Siri, so, is returning as a god in Egyptian mythology (6)
I believe the answer is:
osiris
'a god in egyptian mythology' is the definition.
(I've seen this in another clue)
'siri so is returning' is the wordplay.
'is returning' shows that the letters should be reversed in order.
'siri'+'so'='siriso'
'siriso' in reverse letter order is 'OSIRIS'.
'as' is the link.
(Other definitions for osiris that I've seen before include "one held to be divine" , "Egyptian god of the underworld" , "Egyptian god of the dead" , "Husband of Isis, father of Horus - sis or I (anag)" , "dead judge?" .)